I've also heard that if you overclock the memory on the video card and it detects errors that it will automatically try to "fix itself" which means throttling down performance to prevent a crash- something to do with error detection or correction. I'm not sure if it would lower performance that much but something to think about. I would say try running benchmarking with 3dmark06 or vantage. See if there is an actual improvement with the stock clocks and your overclock settings. If there is no improvement then it may be the memory errors.